1. Field of the Invention
The present invention relates to a method for enabling collaboration among terminals in a wireless network. More particularly, the present invention relates to a method for wireless and fixed terminals of a wireless network to collaboratively communicate with a Base Station (BS) in order to enhance a diversity gain in a wireless network.
2. Description of the Related Art
Wireless communication systems including Mobile Stations (MSs), which are also referred to as mobile terminals, or terminals, are developed to provide wireless communication between users. As technology has advanced, mobile terminals provide an increasing number of features, such as data communications, internet browsing, video conferencing and/or chatting, personal media player functionality, Short Messaging Service (SMS), Multimedia Message Service (MMS), E-mail, games, short range or near field communication, an image capturing function and other similar functions and features. Along with the increasing functionality of mobile stations, there has been a corresponding increase in the number of users of wireless communication systems.
A wireless communication system uses a wireless communication network, wherein a certain geographical region is divided into sub-regions called cells. The MSs in each cell are served by a base station (BS) that transmits information to a particular MS, or a group of MSs, in its cell. The BS transmits information to the MSs using radio signals along a downlink (DL) radio path, and the MSs transmit information to the BS on an uplink (UL) radio path. The wireless communication system may communicate via a Time Division Duplex (TDD) scheme, a Frequency Division Duplex (FDD) scheme, or Orthogonal Frequency Division Multiplexing (OFDM) as a modulation scheme.
Using the FDD modulation scheme, the transmissions on the UL and the DL may be transmitted at the same time interval, but on different frequency bands. Using the TDD modulation scheme, the transmission on the UL and the DL may be transmitted using the same frequency band but during non-overlapping time intervals. Furthermore, with advancements in research and development, the OFDM modulation scheme has become more widely deployed and developed. Using OFDM, an available bandwidth for a radio path, either the DL or the UL path, is divided into a large number of smaller-bandwidth units, referred to as subcarriers, onto which the information to be transmitted is embedded.
By using a plurality of subcarriers, the OFDM modulation scheme is a multicarrier technique and is increasingly used in wireless communication systems due to its robustness to multipath fading and simpler implementation. The number of OFDM subcarriers in an OFDM system is generally selected to be a power of 2, which allows for using a more efficient Fast Fourier Transform (FFT) and Inverse FFT (IFFT) algorithms. The OFDM subcarriers each transmit a respective complex modulation symbol used to carry the digital information transmitted to, from or between users and elements of the mobile communication system using OFDM.
FIG. 1 illustrates an OFDM transmitter according to the related art.
Referring to FIG. 1, the complex modulation symbols, X(k) k=0, 1, . . . , (N−1), are mapped to an IFFT unit 101 of an OFDM transmitter 100. Also illustrated are Guard subcarriers that reduce an amount of Inter-Symbol Interference (ISI), and, upon which no information is transmitted on the guard subcarriers. After respectively performing IFFT operations on the complex modulation symbols and the guard carriers, the IFFT unit 101 provides the time domain signals to a Parallel-to-Serial (P/S) multiplexer 102 in order to multiplex the time domain signals into a serial signal. The serial signal is provided to a Cyclic Prefix (CP) unit 103 which adds CP samples to the serialized time domain signal after the IFFT operation. The resulting sequence, including the CP, is up-converted from a baseband frequency signal into a Radio Frequency (RF) signal using a Digital-to-Analog Converter (DAC)/RF unit 104. The RF signal is then provided to a Power Amplifier (PA) 105 in order to amplify and transmit the radio signal via a transmit antenna 106.
FIG. 2 illustrates an OFDM receiver according to the related art.
Referring to FIG. 2, in an OFDM receiver 200, a signal is received at a receive antenna 201 and is filtered and amplified by a Low Noise Amplifier (LNA) 202. Next, the received signal is down-converted from an RF signal into a baseband signal and is converted from an analog signal to a digital signal by an Analog-to-Digital Converter (ADC)/RF unit 203. The CP samples of the received digital signal are discarded by a CP Removal unit 204, and the serialized signal is demultiplexed by a Serial-to-Parallel (S/P) demultiplexer 205 and FFT operations are performed on the received digital signals by the FFT unit 206 in order to convert the time domain signal into a frequency domain signal. Next, Frequency Domain Equalization (FDE) operations are performed by the FDE unit 207 using channel estimates obtained from received pilots or reference signals and the estimates of the transmitted complex modulation symbols are obtained.
In the wireless communication systems using the OFDM modulation scheme, if MSs in a cell of a BS simultaneously use non-overlapping subcarrier sets for UL transmissions to the BS, then, when the UL transmissions are received at the BS, the transmission from any one MS is rendered orthogonal to the transmission from any other MS. For example, if a MS i uses subcarrier set {Si} for UL transmissions to the BS, wherein the subcarrier sets used by different MSs are non-overlapping, then, the UL transmissions from the MS i on subcarrier set {Si} received by the BS are not interfered with by any of the transmissions to the BS from any of the MSs in the cell of the BS. Similarly, for DL transmissions from the BS to different MSs, if the BS uses non-overlapping subcarriers to make simultaneous transmissions to the different MSs, then any one DL transmission to a MS is orthogonal to another DL transmission meant for any other MS in the cell of the BS.
This property of the OFDM modulation scheme, that is, the use of orthogonal transmissions signals, allows for simultaneous communications between several MSs and the BS on the UL, and between the BS and several MSs on the DL. Furthermore, when the transmission from the BS or the MS is intended for a single MS or BS, then such transmissions are termed unicast or point-to-point transmissions. When the transmission from the BS or the MS is intended for multiple receivers, such transmissions are called broadcast transmissions. Data can also be transmitted as a broadcast transmission, as in the case of a mobile broadcast TV. Another class of transmissions, multi-cast transmissions are point-to-multipoint, but are specialized transmissions designed for a specific set of receivers rather than any receiver receiving the signal.
In addition to the OFDM modulation scheme, wireless communication systems employ multi-hop relaying to enable data transmitted from a source to reach a destination as a unidirectional flow using relay methods and hierarchical deployments of infrastructure stations, such as BSs and Relay Stations (RSs) or other similar network elements of a wireless communication system. Alternatively, multiple infrastructure stations may transmit data in-tandem to a destination MS in order to take advantage of the broadcast nature of the wireless medium. Such relaying techniques are known as cooperative relaying. These multiple transmissions from the multiple infrastructure stations, when coherently combined at the destination MS, provide a cooperative diversity gain to the signal received by the destination MS, and thus, improving signal quality in a mobile communication system.
In a wireless communication system, typically, a distribution of MS locations in a cell or around a BS is hardly uniform, and a density of MSs located around and/or served by a BS changes depending on a physical geography, a network geography, a network topology and other similar factors. For example, in high density urban and/or conurbation areas, there is a higher density of mobiles per square mile as compared to suburban and rural areas. Density of MSs around a BS is also time dependent, wherein cities, and particularly downtown or business district areas including office buildings that are heavily populated during daylight hours, have higher density of MSs per square mile during the day time as compared to the night time. The high amount of geographical or spatial proximity between MSs is referred to as clustering. However, this high density, or clustering of MSs in certain and particular areas is not currently exploited by mobile communication systems. Accordingly, there is a need for an apparatus and a method for providing terminal collaboration in order to exploit the high density or clustering of MSs so as to improve cellular communications and throughput without adding extra hardware or capacity to the network.